The present invention relates to a magneto-optic memory device using a magneto-optic (MO) recording medium such as a magneto-optic disk or the like and, more particularly, to a device having a function to overwrite information.
In recent years, practical use of an information recording device of the read only type or write once type using an optical disk to store a large capacity of information has begun. On the other hand, an information recording device of the information erasable and rewritable type using a magneto-optic recording medium or phase change type medium (hereinafter referred to as the rewritable type) has been increasingly developed. Among the rewritable type recording media, in particular, the magneto-optic recording medium is a recording medium which can be most easily put into practical use with respect to the medium life or reproduced signal characteristics.
However, in the magneto-optic recording medium, it is difficult to perform erasure/rerecording (namely, overwriting) by the overwriting of information, which has been realized in a magnetic disk or the like, thereby causing a large obstacle when a magnetic disk is replaced by a magneto-optic disk. This is because, in the case of the magneto-optic recording medium, the heat of a laser beam spot and the external magnetic field are used to record/erase information, and the recording/erasing operations are switched by switching coils for generating the external magnetic fields, which are arranged at positions apart from the magneto-optic recording medium, or by mechanically changing the direction of a permanent magnet, so that high speed modulation of a large magnetic field cannot be performed, so that the overwriting cannot be performed satisfactorily.
The magneto-optic disk has been disclosed, for instance, in "MAGNETO-OPTIC DISK STORAGE", IEEE Transaction on Magnetics, Vol. MAG-18, No. 6, pages 1233 to 1237, November 1982.
The operations of recording and erasing information onto and from the magneto-optic recording medium will be described in conjunction with the drawings with respect to a magneto-optic disk, as an example. FIG. 1 is a diagram showing a conventional construction of the magneto-optic disk. FIG. 2 is a diagram showing temperature characteristics of a coercive force of a magneto-optic film which is used in the magneto-optic disk. A magneto-optic disk 1 is constructed by coating a magneto-optic recording film 3 onto a transparent substrate 2 and by further covering the film 3 with a protective film 4. Reference numeral 5 denotes a condenser lens as a part of an optical system for emitting a laser beam for recording and reproducing, and reference numeral 6 indicates an external magnetic field generating coil which is used in recording and erasing.
As shown in FIG. 2, as a magneto-optic recording film 3 which has been used conventionally, there is used a film having characteristics such that its coercive force has a high value, and hence magnetization inversion thereof is not easily caused by an external magnetic field at a temperature within the temperature range from -10.degree. C. to 60.degree. C. (hereinafter, referred to as T.sub.ROOM) at which temperature the magneto-optic recording medium is actually used in order to reproduce information. The operations of recording and erasing information will be described hereinbelow in conjunction with FIGS. 1 and 2. It is assumed that the magneto-optic recording film 3 in FIG. 1 has previously been magnetized downwardly and perpendicularly with respect to the film surface before information is recorded. The intensity of the laser beam focused onto the recording film 3 by the condenser lens 5 is enhanced when information is recorded or erased. The temperature of the recording film 3 at a portion thereof to which the laser beam is irradiated is raised to a value near the Curie temperature T.sub.C of about 200.degree. C., for example. As shown in FIG. 2, when the temperature of the recording film 3 rises to a value near the Curie temperature T.sub.C, its coercive force decreases, and the magnetization direction can easily be inverted by an external magnetic field. At the time of recording information, when an upward magnetic field H.sub.REC is given to the recording film 3 by the coil 6 in the above-mentioned state, inversion of the magnetization direction occurs, and a magnetic domain having upward magnetization is formed as an information pit. At the time of erasing information, in a manner similar to the recording operation, the temperature of the recording film 3 is increased to a value near the Curie temperature T.sub.C by the focused laser beam, and, at the same time, a magnetic field H.sub.ERASE in a direction opposite to that applied when recording information is applied as an external magnetic field by the coil 6, thereby erasing information.
The intensity and direction of the magnetic field which is applied by the coil 6 when recording and erasing information and the change in intensity of a spot of the laser beam will be described with reference to the drawings. FIG. 3 is a diagram showing magnetic fields applied upon general reproducing, recording and erasing of a magneto-optic disk and the change in intensity of a laser beam spot. In FIG. 3, no magnetic field is applied when information is reproduced, and the intensity of the laser beam spot is also set to a low level. Next, when information is newly recorded at a portion of the recording film 3 where information has already been recorded, there has conventionally been used a method in which the recorded information at such a portion of the recording film 3 is first erased, and, thereafter, new information is recorded. That is, the magnetic field is set to the erasing direction (H.sub.ERASE), the intensity of the laser beam spot is raised, and the magnetization direction of the information recorded portion is aligned to the magnetization in the erasing direction, thereby erasing the recorded information. After that, the set direction of the magnetic field is inverted to the direction of the magnetic field H.sub.REC in the recording direction, and the intensity of the laser beam spot is modulated in accordance with an information signal to be recorded, so that the magnetization direction of the magneto-optic recording film 3 is partially inverted to thereby record information.
However, with such a conventional recording and erasing method, when new information is overwritten on the recording film 3 on which information has previously been recorded, it is necessary that the previous information first be erased, and that thereafter the laser beam be returned again to the information erased track to record new information. Thus, it is necessary to make the laser beam spot trace and irradiate the same position twice in order to record information there once.
On the other hand, in the case of a magnetic recording medium such as a magnetic disk or the like, it is possible to overwrite information by a single tracing operation, so that the information transfer speed of the magneto-optic disk (abbreviated as the M-O disk) when recording information is half or less of that of the magnetic disk. Therefore, in order to make the M-O disks suitable to replace magnetic disks in application fields in which magnetic disks have conventionally been used, it is necessary for the M-O disks to be improved so as to be provided with an overwriting means which is equal to or better than that employed with magnetic disks and to solve the above-described problem that the information transfer speed is slow in the M-O disks.
As methods of performing overwriting with M-O disks, there have hitherto been proposed a system using magnetic field modulation, a system for inverting the magnetization direction of recording at every rotation of the disk, a system using a magneto-optic disk medium of a double-layer film structure, etc. However, those systems also have respective drawbacks. Particularly, when considering the use of an M-O disk as a data file in place of the magnetic disk, each of the aforesaid systems encounters difficulties when attempted to be put into practical use.
For instance, in the case of the system using magnetic field modulation, when information is recorded, the light intensity is always set to be high, and the direction of the magnetic field which is applied by the coil is changed to the recording direction or the erasing direction in accordance with an information signal. However, there is a limitation when the direction of the magnetic field is changed at a high speed in accordance with the information signal. This is because an electromagnetic coil which is placed at a position apart from the magneto-optic film by several mm is used as means for generating a magnetic field which can be modulated and a magnetic field as large as several hundred Oe is generated on the magneto-optic film. As a result, the number of turns of the electromagnetic coil increases, and the inductance of the electromagnetic coil becomes large, and therefore the modulation of a current flowing through the electromagnetic coil, that is, the modulation of the magnetic field by the information signal having a frequency as high as several MHz, becomes difficult. Further, there has also been made an attempt such that a distance between the magneto-optic film and the electromagnetic coil is reduced to reach a few .mu.m and the number of turns of the electromagnetic coil, which is necessary to generate the same magnetic field, is reduced, thereby enabling the modulation of the magnetic field at a high frequency. However, in the case of a portable type recording medium such as an optical disk, when considering accidental contact between a magnetic head comprising the electromagnetic coil and the magneto-optic recording film, adhesion of dust onto the recording medium, etc., it is considered to be difficult to use the above-mentioned system as a data file which requires high reliability. As mentioned above, it has been difficult to use such a system in an application such as a data file for computers in which a high information transfer speed is required and high reliability is desired at the same time.
In the system which inverts the magnetization direction at every rotation of the disk, two laser beam spots are used, the magnetic field generated by the electromagnetic coil is inverted at every rotation of the disk, and, while recorded information on one recorded track is erased by one of the two laser beam spots, new information is recorded on one adjacent recording track (which has already been erased one rotation before by the method mentioned above) under the inverted magnetic field. However, in such a system, a time period is required to switch the magnetic field at every rotation of the disk; as a result, an area where information cannot be recorded occurs at a position which the laser beam spot has passed during the switching time period at every rotation of the disk. Further, although such a system is effective when continuously recording a relatively large amount of information such as in image or music applications or the like, it is disadvantageous in that it cannot be used in applications requiring high throughput, such as a data file in which one rotation of the disk is divided into several sectors and information is recorded on a relatively small information unit basis.
On the other hand, in the system using the M-O disk of a double-layer film structure which has recently been proposed, one optical head for irradiating a recording medium with a laser beam spot and two external magnetic fields are used. The first layer having a small coercive force is erased only by the first external magnetic field, the magnetic field in the erasing direction for the erased first layer and the magnetic field in the recording direction of the second external magnetic field are selected, and information is recorded in the second layer. Although such a system is capable of overwriting information in principle, the recording medium construction becomes complicated, and besides, a very large magnetic field is required as the first external magnetic field, so that it will be difficult to put it into practical use.
As described above, when considering particularly an application to a data file, the M-O disk conventionally has had a problem such that it is difficult for it to overwrite information.